1. Field of the Invention
The present invention relates generally to a process for removal of sulfur compounds to low levels while minimizing loss of octane. More particularly the invention relates to a process for the removal of mercaptans and thiophenes from a light fluid catalytic cracked naphtha stream.
2. Related Information
Catalytically cracked naphtha gasoline boiling range material currently forms a significant part (≈⅓) of the gasoline product pool in the United States and it provides the largest portion of the sulfur. The sulfur impurities may require removal, usually by hydrotreating, for downstream processing or in order to comply with product specifications or to ensure compliance with environmental regulations.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit:                (1) RSH+H2→RH+H2S        (2) RCl+H2→RH+HCl        (3) 2RN+4H2→RH+NH3         (4) ROOH+2H2→RH+H2O        
Typical operating conditions for the HDS reactions are:
Temperature, ° F.600–780Pressure, psig 600–3000H2 recycle rate, SCF/bbl1500–3000Fresh H2 makeup, SCF/bbl 700–1000After the hydrotreating is complete, the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now desulfurized naphtha.
In addition to supplying high octane blending components the cracked naphthas are often used as sources of olefins in other processes such as etherifications. The conditions of hydrotreating of the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction reducing the octane and causing a loss of source olefins.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. Since the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction the most common solution has been prefractionation prior to hydrotreating. The prefractionation produces a light boiling range naphtha which boils in the range of C5 to about 250° F. for light cracked naphtha (LCN) and a heavy boiling range naphtha which boils in the range of from about 250–475° F. or heavy cracked naphtha (HCN).
The predominant light or lower boiling sulfur compounds are mercaptans while the heavier or higher boiling compounds are thiophenes and other heterocyclic compounds. Fractionation alone of the LCN will not remove the mercaptans. Often, in the past the mercaptans have been removed by oxidative processes involving caustic washing. A combination oxidative removal of the mercaptans followed by fractionation and hydrotreating of the heavier fraction is disclosed in U.S. Pat. No. 5,320,742. In the oxidative removal of the mercaptans the mercaptans are converted to the corresponding disulfides.
In another process the mercaptans in the light cracked naphtha are reacted directly with the dienes contained within the naphtha to form the disulfides. The disulfides may then be subjected to the standard hydrodesulfurization process. However, in the hydrodesulfurization of naphtha which still contains olefins the H2S can recombine with olefins at the reactor outlet to produce mercaptans.
It is an advantage of the present invention that the sulfur may be removed from an LCN stream without any substantial loss of olefins.